Before reaching out, check why the affiliate went quiet. Low conversions, a product change that affected their audience, or simply not having a good creative hook are different problems requiring different approaches. An affiliate who stopped promoting because your conversion rate dropped needs a different email than one who just ran out of content ideas. If you have access to their historical performance data, use it: "You drove 40 sales in Q3. I noticed activity dropped off after that and wanted to check in" is specific enough to earn a real reply.
The reactivation email should give the affiliate a reason to start again: a new product or feature worth covering, updated creative assets, a limited-time commission increase, or simply a fresh angle they haven't tried. "We just launched a campaign analytics dashboard and think it's a strong fit for your audience. Happy to send a brief and some new creative if you'd like to give it a run" gives them something to work with. Affiliates who don't reply to one reactivation attempt are worth one follow-up. After that, focus recruitment energy on new partners rather than chasing dormant ones.